1. Field of the Invention
The invention relates to the field of metal-oxide-semiconductor (MOS) field-effect transistors (FETs) and more particularly, to the gate oxide used in these transistors.
2. Prior Art
While the need for an integrated circuit which operates at two different power supply potentials has been recognized, its practical realization has been another matter. Such an integrated circuit would be useful to drive a bus at one potential while internally operating at another potential. This would permit, for instance, a new microprocessor to operate at a higher potential with older peripherals than the potential used internally for high speed logic circuits. There are numerous other applications where an integrated circuit operating at two potentials would be useful.
The problem in realizing such an integrated circuit is that it requires for efficient operation, two different gate oxide thickness'. No effective, commercially practical method has been found for providing these two different thickness' of oxides on the same substrate. While it is known that silicon nitride may be used to inhibit the growth of an oxide layer thus, for instance, permitting field oxides of different thickness' to be produced such as shown in U.S. Pat. No. 4,352,236, the silicon nitride layers have not been suitable for use with gate oxides. The silicon nitride typically has a high defect density preventing its use as an efficient insulator for the gate of field-effect transistors.